UNIVERSITY  OF  CALIFORNIA 

AGRICULTURAL  EXPERIMENT  STATION 
E.   W.    HlLGARD,   Director 


FHE   OLIVE   KNOT 

BY 

Frederic  T.  Bioletti 

Instructor  in  Practical  Wine-Making  and  Bacteriology 


BULLETIN    No.  120 


•University  Iprees 

BERKELEY 
March,  1898 


THE   OLIVE   KNOT 


By  Frederic  T.  Bioletti. 


The  Olive  Knot  is  a  serious  disease  of  the  olive  tree,  from  which, 
until  lately,  California  seems  to  have  been  exempt.  Its  recent  dis- 
covery in  Merced  County,  however,  makes  it  necessary  that  those 
interested  in  olive  growing  should  be  made  fully  acquainted  with  all 
that  is  known  of  a  practical  nature  regarding  the  disease,  in  order 
that  its  spread  shall  be  restricted  as  much  as  possible.  It  is  quite 
possible,  indeed  probable,  that  the  disease  exists  in  a  mild  form  in 
other  districts,  where  it  is  prevented,  by  the  local  conditions,  from 
increasing  to  a  noticeable  extent  or  doing  any  appreciable  damage. 
There  is  danger,  however,  if  such  districts  exist,  that  they  will  serve 
as  centers  of  infection,  from  which  the  disease  will  spread  to  other 
districts,  where  a  different  set  of  conditions  will  allow  it  to  assume  a 
more  virulent  and  destructive  form.  As  no  satisfactory  curative 
measures  are  known  for  a  tree  once  badly  attacked,  it  is  important 
that  strict  quarantine  measures  should  be  taken.  This  is  especially 
true  in  the  case  of  infected  nurseries,  or  of  olive  groves  from  which 
buds  or  cuttings  are  taken  for  propagation. 

Name  of  the  Disease. — The  disease  which  is  designated  here  as 
Olive  Knot,  is  known  in  Europe  under  many  names,  among  the  com- 
monest of  which  are  Rogna,  in  Italy,  and  Gale,  in  France,  both  words 
about  equivalent  to  the  English  "mange".  In  scientific  publications 
it  is  generally  spoken  of  as  "Tuberculosis  of  the  Olive".  This  name, 
though  very  descriptive,  is  undesirable  for  the  reason  that,  to  some 
people,  it  carries  with  it  the  idea  of  a  connection  between  this  disease 
of  the  olive  and  the  tuberculosis  of  animals  and  man.  Both  diseases 
are  caused  by  bacteria,  but  so  is  the  production  of  vinegar;  there  is 
no  other  connection  between  the  two,  and  no  possibility  of  one  pro- 
ducing the  other.  To  avoid  giving  a  wrong  impression,  then,  it 
seems  best  to  adopt  the  equally  appropriate  and  descriptive  name  of 
Olive  Knot. 

Distribution. — The  Olive  Knot  occurs  in  all  parts  of  Italy,  in 
Egypt,  in  Southern  France,  and  probably  in  all  parts  of  the  olive- 
growing  region  bordering  on  the  Mediterranean.  As  a  rule,  the 
disease  is  worst  in  the  most  southerly  districts,  and  especially  in  those 
where  the  orchards  are  most  thoroughly  cultivated.     The  distribution 


4  UNIVERSITY   OF   CALIFORNIA. 

is,  to  a  great  extent,  sporadic,  both  as  regards  the  whole  Mediter- 
ranean region,  and  as  to  particular  localities.  Some  of  the  most 
seriously  affected  districts  are  widely  separated;  and  in  the  same 
locality  badly  diseased  orchards  lie  side  by  side  with  orchards  that 
are  almost  free.  Occasionally  only  a  few  trees  in  an  orchard  are 
affected,  and  the  disease  remains  for  a  long  time  confined  to  them 
without  showing  any  tendency  to  spread.  That  it  does  spread,  how- 
ever, from  a  center  to  surrounding  trees  and  localities,  like  other 
infectious  diseases,  is  undoubtedly  true  in  most  cases. 

Historical. — Although  this  disease  is  said  to  have  been  known  to 
the  Romans,  and  to  have  been  described  by  both  Theophrastus  and 
Pliny,  it  was  not  until  about  the  middle  of  this  century  that  it 
attracted  any  considerable  attention.  So  long  as  only  the  hardier 
varieties  were  grown,  and  intensive  cultivation  of  the  olive  had  not 
been  adopted,  the  disease  was  limited  in  range  and  comparatively 
harmless.  As  soon,  however,  as  the  finer  and  more  delicate  varieties 
of  olives  were  planted,  and  olive  orchards  were  subjected  to  modern 
methods  of  cultivation,  irrigation  and  manuring,  the  conditions  most 
favorable  to  the  disease  were  established,  and  the  damage  occasioned 
by  the  "Tuberculosis  of  the  Olive"  became  serious. 

The  strong  resemblance  of  the  olive  knots  to  the  galls  produced  by 
insects  on  many  plants  led  most  of  the  earlier  observers  to  believe 
that  they,  too,  were  caused  by  insects.  The  mistake  was  the  more 
easily  made  as  the  galls  are  often  found  pierced  by  insects,  which 
have  used  them  for  food  or  shelter. 

In  1768,  G.  Targioni-Tozzetti,  in  describing  the  olive  trees  of 
Monti  Pisani,  mentions  the  Olive  Knot  and  states  that,  appearing  first 
at  Monte  Morelli,  it  had  spread  over  a  large  part  of  Tuscany.  He 
ascribed  the  disease  to  the  attacks  of  insects.  In  1787,  A.  M.  Fineschi 
describes  an  invasion  of  this  disease  in  the  olive  orchards  surrounding 
Siena,  consequent  on  the  cutting-back  and  manuring  of  the  trees.  In 
3789,  G.  M.  Giovene,  and  in  1790,  C.  Moschettini,  published  in  Naples 
the  first  complete  and  accurate  accounts  of  the  disease.  They  con- 
sidered the  cause  to  be  a  plethoric  condition  of  the  tree.  The  infec- 
tious nature  of  the  disease  has  long  been  known  in  some  parts  of 
Italy,  to  the  peasants  who  prune  the  trees.  In  1817,  C.  Pollini  states 
that  the  primers  believed  that  it  was  possible  to  give  the  disease  to 
healthy  trees  by  pruning  them  with  implements  used  on  affected  trees. 
A.  Costa  (1877),  in  ^ Southern  Italy,  was  the  first  to  show  con- 
clusively that  the  knots  are  not  due  to  insect  attack,  by  pointing  out 
that  there  are  no  cavities  nor  insects  in  most  of  them,  and  that  the 
few  insects  that  are  found  in  them  are  of  various  species. 

In  1886,  Arcangeli  and  Savastano  discovered  the  presence  of  a 
species  of  bacterium  in  the  incipient  olive  knots.  The  former  named 
it  Bacterium  oleae,  but  did  not  believe  it  to  be  the  cause  of  the  disease. 
Savastano  was  the  first,  in  1887,  to  prove,  by  means  of  inoculations, 
that  the  bacterium  is  the  real  inciting  cause.  He  made  a  very  thorough 
and  complete  study  of  the  knots  and  of  the  bacterium.  The  following 
illustration  is  a  reproduction  of  a  drawing  by  Cavara,  showing  the 
microscopic  appearance  of  a  section  of  one  of  the  knots,  and  also  of 
the  bacterium,  which  is  now  called  Bacillus  oleae. 


THE    OLIVE   KNOT. 


FIG.    1. 

\         X      ' 

Section  through    a  young  knot,   showing  "callus-  I 

like"    hypertrophied    tissue    and    the  central  hollow        v    _,._ 
containing  the  colony  of  bacteria.    Below  it  is  shown 
the  microscopic  appearance  of  the  Bacillus  oleae. 


-    /     \ 


Occurrence  of  the  Disease  in  California  :  —Olive  Knot  was  first 
noticed  in  California  in  an  orchard  near  the  town  of  Merced.  Mr.  A. 
R.  Gurr  first  observed  it  in  1893  on  a  single  tree.  Since  then  it  has 
been  gradually  spreading,  until  now  it  has  destroyed  several  trees  and 
has  spread  over  a  large  part  of  the  orchard. 

A  visit  was  made  to  Mr.  Gurr's  place  in  February,  1898,  with  the 
object  of  investigating  the  disease  on  the  spot. 

The  orchard  is  on  a  rich  sandy  soil  near  an  irrigating  canal  and  is 
thoroughly  sub -irrigated.  The  water  level  is  said  to  be,  on  the  average, 
about  five  feet  from  the  surface .  The  trees  grow  very  vigorously  and  bear 
well.  They  were  not  pruned  much  until  attacked  by  the  disease,  when 
pretty  severe  cutting-back  was  resorted  to  with  the  idea  of  removing 
the  diseased  parts.  The  affected  trees  immediately  surround  those 
first  attacked,  while  the  part  of  the  orchard  farthest  removed  from  the 
point  of  infection  is  yet  free  from  the  disease.  The  following  plan  of 
the  affected  orchard  (fig.  2),  drawn  from  data  furnished  by  Mr.  Gurr, 
shows  graphically  how  the  disease  has  spread  from  the  tree  first 
attacked  (a)  to  the  surrounding  trees. 

Some  large  trees  were  found  to  be  literally  covered  with  knots. 
They  occurred  on  the  main  trunk,  branches  and  twigs,  and  were 
particularly  abundant  wherever  the  tree  had  been  cut  or  otherwise 
injured.  These  trees  showed  the  effect  of  the  disease  in  limited 
growth,  scanty  foliage,  and  occasional  dead  branches,  and  were 
evidently  on  the  point  of  succumbing.  Other  large  trees,  which  had 
not  been  affected  long,  showed  immense  numbers  of  knots  on  the 
small  twigs,  but  very  few  on  the  trunk  and  branches.  These  trees 
looked  as  vigorous  and  healthy  as  the  unaffected  trees  and  had  borne 
a  large  crop.  Finally,  on  the  edges  of  the  affected  area  the  trees 
showed  only  a  few  knots,  which  in  nearly  all  cases  were  on  the  small 
twigs. 


UNIVERSITY   OF    CALIFORNIA, 
COUNTY    ROAD. 


(<§})    (<§j) 
((§))    (<§>) 


(P  TREES    SERIOUSLY    INJURED. 

©  TREES   SHOWING    NUMEROUS    KNOTS. 


O  TREES   SHOWING   A    FEW    KNOTS. 


o    o 


o    o    o 


o    o    o    o    o    o 
O  '  o    o    o    o    o 


FIG.   2.— PLAN    OF  THE  AFFECTED   ORCHARD. 

A .  The  first  tree  on  which  the  knots  were  noticed. 

B.  Tree  showing  an  immense  number  of  knots. 

C.  Mission  Olive  almost  free  from  knots. 

D.  Redding  Picholine  almost  free  from  knots. 

There  was  little  evidence  on  which  to  base  a  judgment-  as  to  the 
various  degrees  of  susceptibility  of  the  various  varieties  grown  here. 
The  Columbella  seemed  to  be  particularly  susceptible,  most  of  the  dead 
trees  being  of  that  variety,  but  this  may  be  due  simply  to  the  fact  that 
they  happened  to  be  in  the  area  first  attacked.  The  Mission  and 
Redding  Picholine  seemed  to  be  comparatively  resistant,  as  far  as  could 
be  judged  from  a  single  tree  of  each  variety  growing  next  to  badly 
diseased  trees  on  the  edge  of  the  affected  area  (see  (7,  D,  fig.  2).  In 
general,  the  old  trees  were  attacked  principally  on  the  younger  parts. 
Correlated  with  this  is  the  fact  that  young  trees  were  evidently  more 
susceptible  than  old,  and  suffered  more  injury  from  the  attack.  Young 
trees  planted  to  replace  trees  destroyed  by  the  disease  were  in  all  cases 
literally  covered  with  tubercles  {see  plate  3)  and  in  some  cases  killed. 
The  tree  represented  in  the  figure  had  over  ninety  knots. 

Wherever  a  jagged  wound,  such  as  that  made  by  tearing  off  a 
branch,  or  by  a  blow  from  the  plow,  was  seen,  the  tubercles  were  very 
numerous.  The  edge  of  such  a  wound  is  evidently  an  excellent  place 
for  infection  {see  plate  5) .  The  smooth  wounds  made  in  pruning 
were  much  less  severely  or  generally  infected. 

The  roots  seemed  little  affected.  A  small  tree,  which  was  a  mass  of 
tubercles  above  ground,  showed  only  one  about  the  size  of  a  pea  on  the 
roots  {see  plate  3) . 

Occasionally,  small  tubercles,  varying  from  the  size  of  a  pinhead 
to  that  of  a  pea,  occurred  on  the  leaves,  generally  on  the  under  side. 


A 


* 


i 


\ 


gv 


?*9 


' 


v 


"W- 


f  #  /  \  I 


•V, 


1. — Young  tree   planted  to   replace   one   destroyed    by    knot.     Photograph    taken 
twelve  months  after  planting. 

2.— Large  knots  on  branch  of  old  tree. 

3. — Small  knots  on  weakened  branch. 


1.— Small  knots  on  under  side  of  leaves. 

2. — Large  knots  on  small  branch,  as  commonly  seen  on  vigorous  trees  when  first 
attacked. 

3.— Knot  on  leaf-stalk. 


PLATE    5. 


lie  to  general  infection  around  the  edges  of  a. 
2. — Large  swelling  or  "uovoli"  on  main  trunk  infected  with  olive  knot. 


1.— Mass  of  hypertrophied  ti 
ragged  wound  made  by 


THE    OLIVE    KNOT.  I 

The  occurrence  of  affected  leaves  was  very  irregular,  most  trees  show- 
ing none.  When  they  did  occur  they  were  generally  confined  to  one  or 
two  small  branches,  and  even  then  were  not  numerous.  It  was  only 
rarely  that  nearly  all  the  leaves  on  a  small  branch  exhibited  tubercles. 
The  number  of  knots  on  the  leaves  seemed  to  bear  no  relation  to  the 
severity  of  the  attack  on  the  rest  of  the  tree.  Some  of  the  most  badly 
affected  trees  showed  no  excrescences  on  the  leaves. 

What  fruit  remained  on  the  trees  was  too  badly  frost-bitten  for 
judging  whether  or  not  it  had  been  invaded  by  the  bacteria.  Mr. 
Gurr  said,  however,  that  the  olives  on  diseased  trees  seemed  affected, 
and  that  an  attempt  to  pickle  them  failed,  as  they  decayed  before  the3r 
were  ready  for  the  brine. 

Nature  of  the  Disease. — Savastano*  describes  the  Tuberculosis  of 
the  Olive  as  follows:  "The  tubercles  which  characterize  this  disease 
are  generally  formed  on  branches  of  from  one  to  fifteen  years  of  age, 
where  the  bark  is  still  smooth  and  not  cracked.  They  are  generally 
isolated,  but  sometimes  take  the  miliary  form.  They  are  rare  on  the 
roots,  and  still  more  so  on  the  leaves  and  fruit.  So  far  I  have  not 
found  them  on  the  flowers,  but  believe  they  may  occur  there. 

"The  formation  of  a  tubercle  takes  place  in  the  following  manner: 
Generally,  quite  close  to  the  cambium  layer,  and  most  frequently  in 
the  bast,  a  colony  of  bacteria  commences  to  form.  To  the  unaided  eye 
it  appears  as  a  small  transparent  spot.  With  a  magnification  of  1000 
diameters  this  is  seen  to  be  a  colony  of  bacteria.  At  the  same  time 
a  hypertrophy  of  more  or  less  degenerated  elements  commences  to  form 
around  this  colony.  The  colony  increases  in  size  and  becomes  dark- 
colored.  The  hypertrophy  increases,  causing  the  tubercle  to  enlarge 
and  finally  burst  through  the  cortex  and  split.  A  tubercle,  thus 
formed,  continues  to  grow  each  year  more  or  less  until  it  attains  a 
diameter  of  from  one  to  two  centimeters.  The  tubercles  commence  to 
form  in  the  spring;  during  the  heat  of  summer  the  hypertrophy  is 
arrested,  but  the  colony  of  bacteria  continues  to  increase.  Later, 
during  the  autumn  vegetative  growth,  the  hypertrophy  recommences. 
In  the  fruit  there  is  no  true  hypertrophy." 

Prillieux  explains  the  formation  of  the  knots  under  the  influence 
of  bacteria  as  the  natural  effort  of  growing  parts  of  the  plant  to  heal 
injuries.  Callus  or  healing  tissue  forms  on  the  first  attack  of  the 
bacteria  in  the  same  manner  as  when  the  plant  receives  a  mechanical 
wound.  This  tissue  being  soft  is  immediately  attacked  by  the  increas- 
ing colony  of  bacteria,  and  fresh  callus  is  formed  until  the  twig 
attacked  becomes  too  weak  to  form  more,  when  the  further  enlarge- 
ment of  the  knot  comes  to  an  end. 

Savastano  investigated  the  influence,  on  this  disease,  of  cuts,  bruises, 
pruning  and  other  mechanical  injuries  to  the  plants,  and  came  to 
the  following  conclusions: 

"(1)  Wounds  do  not  cause  tubercles  except  on  plants  which  are 
already  predisposed  to  the  disease.  (2)  Even  in  these  plants  a  wound 
does  not  always  cause  the  formation  of  tubercles.  (3)  Heavy  pruning 
causes  the  formation  of  new  tubercles,  and  an  increase  in  size  of  the 


*Comptes  Rendus  de  l'Academie  des  Sciences;  CIIL,  1144. 


8  UNIVERSITY   OF    CALIFORNIA. 

old  on  affected  trees.  (4)  The  formation  of  tubercles  is  in  direct  ratio 
to  the  vigor  of  the  plant." 

Savastano' s  researches  show  that  some  varieties  of  olive  are  more 
susceptible  to  the  disease  than  others,  but  that  all  varieties  may  con- 
tract the  disease  when  the  conditions  are  those  which  most  favor  its 
growth.  The  most  susceptible  varieties  are  unfortunately  the'  heaviest 
bearers,  and  those  with  the  finest  fruit.  The  conditions  which  pre- 
dispose the  tree  to  the  disease  are :  rich  soil ;  excessive  irrigation  or 
rain;  heavy  manuring;  thorough  cultivation;  severe  pruning.  Any- 
thing, in  fact,  which  tends  to  produce  vigorous  vegetation  or  high  sap 
pressure  tends  to  make  the  tree  more  susceptible. 

The  manner  in  which  the  tree  becomes  infected  with  the  disease  is 
not  known  positively  in  all  cases.  Anything  which  wounds  the  bark, 
such  as  cuts,  bruises,  hail,  frost,  pruning,  grafting,  budding,  etc.,  is 
often  followed  by  a  growth  of  tubercles,  as  it  allows  the  entrance  of 
bacteria.  The  disease  occurs  sometimes,  however,  in  places  where 
there  has  apparently  been  no  mechanical  injury  to  the  tissues  of  the 
tree.  It  is  quite  possible  that  biting  and  sucking  insects  may,  in 
piercing  the  bark,  inoculate  the  tree  with  the  disease.  According  to 
Vuillemin,  a  certain  fungus  is  always  found  associated  with  the  Olive 
Knot.  This  fungus,  he  claims,  penetrates  the  tissues  of  the  tree  and 
opens  the  way  for  infection  by  the  bacteria. 

That  the  disease  is  infectious,  and  is  caused  by  bacteria,  there 
seems  to  be  no  doubt.  Savastano  states :  (1)  That  a  colony  of  bacteria 
is  to  be  found  in  the  earliest  stage  of  the  formation  of  all  tubercles. 
(2)  That  he  found  the  same  species  of  bacterium  in  all  the  hundreds 
of  tubercles  he  examined.  (3)  That  the  spread  of  the  disease  from  a 
first  infected  center  to  surrounding  districts  has  often  been  observed. 

Later,  Savastano  made  pure  cultures  of  the  bacterium,  and  in- 
oculated many  plants  both  with  this  and  other  kinds.  The  results 
obtained  were  that  inoculation  with  the  tubercle  bacterium  nearly 
always  produced  tubercles  on  the  olive,  but  never  on  other  plants, 
while  inoculations  of  many  other  kinds  of  bacteria  never  produced 
tubercles  on  the  olive,  or  only  so  rarely  as  to  be  accounted  for  by 
accidental  infection. 

Symptoms  of  the  Disease. — A  complete  account  of  the  effects  of 
the  disease  on  the  trees  cannot  be  given  at  present,  as  only  one  visit 
has  been  made  to  the  affected  orchard.  At  other  seasons  of  the  year 
other  symptoms  may  be  shown.  The  disease  has  no  apparent  effect  on 
the  general  health  of  the  tree  until  it  becomes  literally  a  mass  of  knots. 
It  is  but  rarely  that  a  branch  or  twig  is  killed  while  the  rest  of  the 
tree  remains  healthy,  and  it  is  not  until  the  last  stage  is  reached  that 
there  is  any  diminution  of  vigor  or  fertility  in  the  affected  tree.  The 
symptoms,  therefore,  may  be  said  to  be  confined  to  the  various  kinds 
of  excrescences  which  appear  on  the  different  parts  of  the  tree,  and  a 
description  of  these  will  suffice  to  characterize  the  disease. 

The  knots  on  the  leaves  are  always  small,  especially  when  they  are 
numerous.  They  vary  from  the  size  of  a  pin-head  to  three-sixteenths 
of  an  inch  in  diameter.  They  occur  almost  always  upon  the  lower  sur- 
face only,  but  show  through  on  the  upper  surface  as  brown  spots. 
This  shows  that  the  leaf  tissue  is  killed  quite  through  the  leaf.     On 


THE    OLIVE   KNOT.  9 

the  leaf  stalks  the  knots  are  generally  larger,  say  one-fourth  to  three- 
sixteenths  of  an  inch  (see  plate  4) . 

On  the  small  twigs  of  vigorous  trees  the  knots  often  measure  one 
inch  across,  and  become  much  contorted  and  split  up  by  drying-out 
and  the  pressure  caused  by  the  continuous  growth  of  new  tissue  within 
and  at  the  base  (see  plate  4) .  However  large  a  knot  becomes  it  seldom 
or  never  involves  the  whole  circumference  of  a  twig.  On  the  side 
opposite  the  knot  the  bark  seems  healthy.  This  accounts  for  the  fact 
that  a  tree  remains  green  and  fresh-looking  for  so  long  a  time  after 
being  severely  attacked.  Occasionally,  especially  on  young  and  badly 
affected  trees,  several  points  of  infection  will  coalesce.  In  this  case 
the  bark  and  growing  layer  of  the  twig  maj^be  killed  all  around.  Still 
more  rarely  the  infection  seems  general  on  a  certain  part  of  the  twig, 
and  for  an  inch  or  more  there  will  be  a  kind  of  breaking-out,  owing  to 
a  general  hypertrophy  of  the  tissues,  not  arising  from  any  definite 
points  of  infection,  and  not  producing  definite  knots.  This  is  shown 
somewhat  imperfectly  just  at  and  below  the  point  of  branching  by  the 
young  tree  in  the  middle  of  plate  3.  The  last  two  cases  account  for  the 
occasional  dead  twigs  and  small  branches  which  occur.  In  some  cases 
where  there  is  a  large  number  of  points  of  infection,  or  when  the  tree 
is  less  vigorous,  the  knots  remain  small,  not  exceeding  one  half  inch 
in  diameter.  They  are  also  more  regular,  more  nearly  spherical,  and 
do  not  develop  so  many  cracks  nor  such  an  uneven  surface  (see  right 
side  of  plate  3  J. 

On  trunks  and  branches  the  knots  finally  attain  a  large  size,  often 
becoming  as  much  as  two  inches  in  diameter.  This  seems  to  be  a 
larger  size  than  they  attain  in  Europe,  judging  from  descriptions. 
They  are  flatter,  that  is,  more  spreading  and  less  spherical,  than  on 
the  twigs.  Occasionally  several  or  many  points  of  infection  occur 
near  together,  and  the  resulting  tubercles  run  together,  thus  forming 
a  large  mass  of  cracked,  diseased  tissue  several  inches  across.  This 
is  very  commonly  the  case  in  the  infection  of  ragged  wounds,  where 
the  points  of  infection  are  so  numerous  that  no  definite  knots  are 
formed  ( see  plate  5 J .  The  knots  occur  very  often  on  the  old  wood 
near  the  bases  of '  small  twigs  coming  from  adventitious  buds.  They 
occur,  however,  in  all  situations,  and  infection  appears  possible  any- 
where. 

Some  of  the  trees  on  Mr.  Gurr's  place  which  showed  a  very 
large  amount  of  olive  knot  were  very  heavily  loaded  with  fruit. 
Several  of  the  older  writers  make  the  observation  that  trees  attacked 
by  this  disease  bear  more  than  they  did  before.  The  reason  of  this  is 
that  plants  with  too  much  vegetative  vigor  do  not  bear  well,  and  that 
the  first  attack  of  the  disease  by  slightly  diminishing  this  vigor  makes 
the  trees  more  fruitful.  As  the  disease  progresses,  however,  the 
diminution  of  vigor  continues  until  the  plant  is  unable  to  produce 
either  fruit  or  foliage. 

There  are  several  kinds  of  knots,  or  swellings,  common  on  olive 
trees,  which  might  carelessly  be  confused  with  the  real  olive  knot.  First 
of  all  there  is  the  common  callus  or  healing  tissue  of  wounds  which 
forms  around  cuts,  grafts,  and  inserted  buds.  This  forms  in  a  rather 
thin  layer  around  the  edge  of  the  wound,  is  of  different  shape,  and 
never  reaches  the  size  of  the  true  olive  knot.     The  large    swellings, 


10  UNIVERSITY   OF    CALIFORNIA. 

"uovoli,"  commonly  found  on  the  trunk  of  the  olive  tree,  especially 
near  the  ground,  are  also  of  a  very  different  nature.  They  are  per- 
fectly normal  and  do  not  indicate  a  diseased  condition.  They  are  in- 
deed used  in  Europe  for  the  propagation  of  the  olive.  They  seem, 
however,  to  be  particularly  favorable  places  for  infection  and  for  the 
growth  of  the  true  knot  ( see  plate •  5,  right  side  J.  Finally,  old  olive 
trees  often  develop  tumors  on  the  main  roots,  trunk,  and  larger 
branches,  the  cause  of  which  is  not  well  known.  They  are  flatter, 
less  cracked,  and  generally  grow  to"  a  larger  size  than  the  tubercles 
of  the  Olive  Knot.  They  do  not  seem  to  be  infectious,  nor  to  do  much 
harm  to  the  tree. 

Experiments  in  the  Laboratory. — Most  of  the  knots  at  this 
season  of  the  year  (February)  are  old,  dry  and  cracked,  and  probably 
do  not  contain  living  bacteria.  A  few  knots  were  found,  however, 
which  were  still  succulent  and  unbroken.  From  the  interior  of  these, 
inoculations  were  made  into  the  usual  bacteriological  culture  media. 
Of  seventeen  inoculations  made,  eleven  showed  no  growth.  One  showed 
growth  of  one  kind  of  bacterium  and  a  mold.  Five  showed  growth  of 
another  kind  of  bacterium.  The  last  was  morphologically  the  same  as 
Arcangeli's  bacterium,  as  far  as  indicated  by  the  limited  investigations 
so  far  made.  No  bacteria  could  be  found  by  simple  microscopical 
examination.  This  was  to  be  expected,  as  at  this  season  of  the  year 
•they  are  probably  much  reduced  in  numbers,  and  in  a  dormant 
condition . 

Conclusions  :  — The  Olive  Knot  disease  is  at  present  limited  to  a 
small  range  in  California,  but,  as  it  promises  to  be  very  harmful  in 
some  localities,  special  precautions  should  be  taken  to  prevent  its 
spread. 

The  one  necessary  condition  for  the  existence  of  the  disease  is  the 
presence  of  the  specific  germ,  the  special  bacterium  which  causes  it. 
If  this  can  be  kept  away,  no  other  condition  will  bring  about  the 
disease. 

Even  if  the  disease  germ  be  present,  the  trees  will  not  be  attacked, 
or  attacked  only  slightly,  if  the  conditions  favoring  the  growth  of  the 
germ  do  not  exist  also. 

The  conditions  favorable  to  the  growth  of  the  bacteria  and  to  the 
production  of  knots  are:  (1)  Delicacy  of  the  tissues,  owing  to  youth 
or  variety.  (2)  High  sap  pressure,  due  to  heavy  pruning,  irrigation, 
manuring.  (3)  Wounds,  caused  by  pruning,  grafting,  budding, 
gathering  the  fruit  with  rakes,  and  by  injuries  due  to  insects,  fungi, 
wind,  the  plow,  etc.  (4)  Hot  weather.  Under  the  conditions  most 
favorable  to  the  growth  and  spread  of  the  disease,  it  may  be  con- 
tracted by  any  variety.  Some  varieties,  however,  are  much  more 
susceptible  than  others.  Which  varieties  these  are  cannot  be  exactly 
stated  as  yet,  at  least  for  California.  In  general,  however,  those  which 
are  most  "highly  bred,"  that  is,  which  are  farthest  removed  from  the 
hardy  wild  type,  are  most  liable  to  contract  the  disease,  and  are  most 
vitally  injured  by  it.  These,  unfortunately,  are  precisely  the  varieties 
which  are  most  valuable  for  the  quantity  and  quality  of  their  crops. 

No  buds  nor  cuttings  should  be  taken  from  infected  orchards.  All 
pruning-shears,    saws,   gathering  boxes,   etc.,  used  on  affected  trees 


THE    OLIVE    KNOT.  II 

should  be  thoroughly  disinfected  before  being'  used  anywhere  else. 
This  can  be  done  by  boiling  in  water,  or  by  steeping  in  a  solution  of 
one  part  of  corrosive  sublimate  to  five  hundred  of  water,  for  half  an 
hour  or  longer. 

Where  there  is  danger  of  infection,  all  considerable  wounds  should 
be  painted  with  an  antiseptic  and  protective  paint. 

When  a  tree  is  first  affected  all  diseased  parts  should  be  carefully 
cut  away  and  the  parts  burned  on  the  spot. 

What  pruning  is  necessary  should  be  done  gradually;  that  is,  a 
little  each  year.  This  is  not  so  liable  to  produce  a  tendency  to 
contract  the  disease  as  a  heavy  pruning  every  few  years. 

Where  possible,  the  presence  of  too  much  moisture  in  the  soil 
should  be  avoided. 

Olives  from  diseased  trees  are  perfect^  wholesome,  and  probably 
as  good  for  oil  as  from  healthy  trees,  unless  the  vitality  of  the  tree  is 
seriously  affected.  If  the  fruit  itself  is  attacked,  it  is  probably  useless 
for  pickling. 


LIST  OF  THE  MOST  IMPORTANT  WRITINGS  ON  THE  OLIVE  KNOT. 

Giovene,  Giuseppe  Maria:  Sulla  Eogna  dcgli  Ulivi     Naples,  1789. 
Moschettini,  Cosimo:  Delia  Bogna  degli  Ulivi.     Naples,  1790. 
Arcangeli:,  G. :    Sopra  la  vialattia  delV  Olivo  delta  volgarmente  Bogna.   Pisa,  1S86. 
Savastano,  L. :   Tubercolosi,  Iperplasie  e  Tumori  delV  Olivo.     Naples,  1887. 
Savastano,  L. :     Les  Maladies  de  V Olivier  of  la  Tubereulose  en  particulier.    Comptes 

rendus  de  l'Academie  des  Sciences,  T.  CIII,  p.  1144. 
Savastano,  L. :    Les  Maladies  de  P  Olivier;  hyperplasies  et  tumeurs.     L.  c.  T.   CIII, 

p.  1278. 
Savastano,  L. :  Esperimenti  di  inoculasioni  del  batterio  della  Tubercolosi  delV  Olivo. 

Boll.  Societa  Naturalisti  Napoli,  Ser.  I,  vol.  I,  p.  117. 
Savastano,  L. :  II  Baeillo  della  Tubercolosi  delV  Olivo.     Rome,  1889. 
Prillieux:  Les  Tumeurs  a  Bacilles  de  V Olivier,  etc.  Comptes  rendus  de  l'Acad.  des 

Sciences,  T.  CVIII,  p.  249. 
Briosi  e  Cavara:  i"  Funghi  Parassiti,  fasc.  V,  No.  101.     Pavia,  1890. 
Pierce,  Newton  B.:   Tuberculosis  of  the  Olive.  Journal  of  Mycology,  Vol.  VI,  No. 

4,  1891. 
Vuillemin,    P. :    Association  du   Chaetophoma  oleacina  et  du  Bacillus  oleae.     Bull 

Soc.  Mycol.  de  France.     T.  XIII,  p.  44,  1897. 


